1. Field of the Invention
The present invention relates to optical signal processing. More specifically, the present invention relates to a method and apparatus for enabling high-resolution optical spectrum analysis or channelization, and for enabling related signal processing operations.
2. Background of the Invention
The widespread use of optical communications and sensing has both fueled and benefited from new and better devices for placing signals on and detecting them from light (or optical carriers). The improved optical system performance, along with inherent advantages of optical signals (e.g., size, low loss, bandwidth, immunity to electrical interference, etc.), has led to a natural progression in need for and advantage of processing optical signals. The types of signals to be processed can be digital or analog; may include various types of information such as audio, video, image, data, radar, and other signals; and may exist at various data rates, bandwidths, protocols, or optical modulation types. Here, signal processing includes but is not limited to conventional filtering, multiplexing, coding, routing, analyzing, correlating, and synthesizing signals. With the gaining prevalence of signals in the optical form, new methods, techniques, and devices are needed to make these signal processing functions, which typically utilize electrical or other techniques, readily available for processing optical signals.
Typically, signal processing functions rely on some form of spectral (frequency-based) separation, combination, or both. Since optical systems are typically reversible, the discussions herein are mainly limited to the spectral separation (or spectral analysis) of a signal and treatment of spectral combination or recombination is obtained by reversing the direction of optical signal travel. The manner by which the information is spectrally separated (or analyzed or channelized) is critical to the basic performance parameters and the facilitating of processing functionality. The critical parameters of spectral analyzers are: high throughput efficiency (or low loss), low crosstalk (or clear separation between channels), free spectral range, narrow (or fine) frequency resolution, ratio of frequency range to frequency resolution (also known in the art as the time-bandwidth product), high linearity with frequency (analysis independent of frequency), large number of taps, large number of channels, size, manufacturability, and tight control on internal intermediate signals. Many of these parameters are closely related to each other. An important principal fuctionality of analyzers to be used for higher-level applications is the ease of access to spectrally or temporally resolved signals and ease by which the optical signals interact with other optics. The present invention substantially enhances the performance parameters and the ease of signal access from prior art.
There is a variety of optical devices in the prior art for performing spectrum analysis or channelization of optical signals. Spectrum analyzers typically separate or split the signal into spectral parts and make them available for processing, detection, or recombination. Whereas channelizers typically separate an input frequency band into specific channels, that is, they first analyze and then recombine signals into a plurality of output frequency bands. These devices achieve varying degrees of spectral resolution, crosstalk, and applicability to signal processing depending on their particular design. A selection of such devices is described below.
Fabry-Perot Interferometer
The Fabry-Perot interferometer is a known device for separating light into its component frequencies, or equivalently, its component wavelengths. FIG. 14 illustrates one example of a prior art Fabry-Perot interferometer. The illustrated device comprises two mirrors M1 and M2. Each of the two mirrors M1 and M2 is a partially reflecting mirror. The mirrors M1 and M2 are typically separated by an air space. Alternatively, the Fabry-Perot interferometer device could be made by coating both sides of a transparent plate with a partially reflecting material.
Light from a spectrally broadband source is input at plane S1. Light rays at an angle θ and a wavelength λ undergo multiple reflections between mirrors M1 and M2. The light rays interfere constructively along a circular locus P2 in the output plane S2. The condition for constructive interference that relates a particular angle θ and a particular wavelength λ is given by2d cos θ=mλ,
where d is the separation of the partially reflecting surfaces, and m is an integer known as the order parameter. The Fabry-Perot interferometer thereby separates the component frequencies of the input light by using multiple beam reflection and interference. It is apparent from the equation above that the output light pattern of the system, i.e., the interference fringes, in the case of a diverging input beam, is a set of concentric circular rings. One ring is present for each combination of wavelength component of the input light and each integer m. For any given ring, the ring diameter increases as the light frequency is increased.
The Fabry-Perot interferometer is not well-suited for use in certain spectrum analysis or channelization applications due to the difficulty in obtaining high optical throughput efficiency. If the input beam is divergent, e.g., the direct output of an optical fiber, then the output pattern for a given wavelength is a set of rings. Multiple wavelengths produce nested sets of concentric rings. It is difficult to collect this light efficiently and concentrate it at multiple detector points, or couple it to multiple output fibers, especially while maintaining the separation of wavelength components that the interferometer has produced. If the input beam is collimated, e.g., the collimated output of an optical fiber, then the beam can be confined to a narrow range of angles to produce only a single-order output (e.g., m=+1) for each wavelength of interest. This collimation makes it easy to concentrate the output light at multiple detector points or fibers, but there is inherently high loss. The throughput efficiency can be no greater than 1/N, where N is the number of resolvable wavelength components at the optical system output aperture. That is, for a single wavelength input, only 1/N of the input power is resonant for maximum throughput. The other (N−1)/N fraction of the input is effectively reflected off the Fabry-Perot interferometer back towards the input. Fabry-Perot interferometers have highest throughput when the input beam is well collimated and only a single (or narrow band of) wavelength is being selected or separated. In addition, the fall-off (or “skirts”) of the optical fringes formed by the Fabry-Perot interferometer is relatively large, which limits the crosstalk and channel separation of the device.
OTDL Channelizer
FIG. 15 shows an example of a prior art planar waveguide integrated optical multiplexer and demultiplexer device, as disclosed by Bhagavatula in U.S. Pat. No. 6,111,674. In this device, a multiple-wavelength input signal is demultiplexed or channelized using a Fabry-Perot thin film stack consisting of alternating partially reflective and transmissive layers. The angularly dispersed wavelengths emerge from the “optical path length difference generator” and are individually coupled to a fan-shaped output array of waveguides by means of a focusing lens. This device could be fabricated as either a planar or a hybrid integrated optical (IO) structure. The drawback to this type of integrated optical demultiplexer/multiplexer is the inherently high loss associated with the thin film wavelength-separation elements, which limits the number of channels that can be effectively channelized. In addition, the spectral resolution is limited by the relatively short optical path length that can be achieved in a planar or hybrid IO structure.
Optical Fiber-Based OTDL Spectrum Analyzer
Ranalli in WO 01/93465 A1 and FIG. 2 therein teaches an optical fiber-based optical tapped delay line spectrum analyzer as replicated here as FIG. 16. The output fiber lines 76 are cut such that adjacent fibers differ in length by about 1 centimeter (cm), which corresponds to a relative delay, T, of about 50 picoseconds (psec) between two optical signals in two adjacent output fiber lines. This delay between adjacent outputs determines the sampling interval for the diffractive array 68. The inverse of the sampling interval (i.e., 1/T) establishes the free spectral range provided by the array 68. In this embodiment, the free spectral range is about 20 GHz. To satisfy the Nyquist sampling theorem and to avoid aliasing, the optical bandwidth of the signal should be less than half the free spectral range; thus, the bandwidth of the optical signal should be less than about 10 GHz. The spectral resolution provided by the array equals the free spectral range divided by the number of taps or output fiber lines 76 into which the optical signal is efficiently coupled. Since the diffractive array 76 produces eight time-delayed outputs, the spectral resolution obtained is about 2.5 GHz. This resolution is much less than the resolution obtained by preferred embodiments of the present invention. Additionally, fabrication and stability of this device may be difficult as environmentally-independent, very precise fiber lengths are needed for proper operation.